It is known that conventional types of ferritic stainless steel nickel in a smaller content than that in austenitic stainless steel, and, therefore, are cheap and exhibit a satisfactory accuracy upon being shaped and no stress corrosion cracking. Therefore, ferritic stainless steel is widely used for producing various kinds of kitchenware and parts for automobiles. However, it is also known that conventional ferritic stainless steel exhibits poor formability (deep drawability, capability of being shaped) compared with the austenitic stainless steel. Also, recently, the source of supplies of nickel is becoming exhausted. Therefore, it is strongly desired by the stainless steel industry to provide a new type of ferritic stainless steel having excellent formability and reduced nickel content.
In the past, many attempts have been made to provide new types of ferritic stainless steel having the above-mentioned properties.
For example, in order to enhance the formability of the ferritic stainless steel, Japanese Examined Patent Publication No. 51-44888 provided an aluminum-containing ferritic stainless steel and Japanese Unexamined Patent Publication No. 51-98616 provided an aluminum-titanium-containing ferritic stainless steel. It is true that the addition of a certain amount of an additional alloy component consisting of aluminum alone or aluminum and titanium to a typical ferritic stainless steel base, that is, a 17% chromium ferritic stainless steel (SUS 430 type), is effective for increasing formability, for example, deep drawability. Addition of the amount of the additional alloy component beyond a certain level, however, fails to have any effect. Also, the effect of the addition of aluminum alone or aluminum and titanium is unsatisfactory.
In other attempts, Japanese Examined Patent Publication No. 44-736 discloses a boron-containing ferritic stainless steel and Japanese Examined Patent Publication Nos. 47-4786 and 51-8733 a boron-titanium-containing ferritic stainless steel. The addition of boron alone or boron and titanium is effective for enhancing the formability, for example, deep drawability, of ferritic stainless steel. The amount of the boron added in the above-mentioned attempts, is however, relatively large. The resultant ferritic stainless steel exhibits poor resistance to corrosion and hot workability, because some types of boron compounds are deposited in the grain boundary regions. Also, the large amount of boron substantially increases cost of the resultant ferritic stainless steel, making the above-mentioned type of boron-containing ferritic stainless steel practically useless in industry.
In still another attempt, British Pat. No. 1,217,933 discloses another type of boron-containing ferritic stainless steel. However, this type of boron-containing ferritic stainless steel contains molybdenum, nickel, and cobalt. Here, the addition of boron is intended to improve the surface quality of the primary ferritic stainless steel material, not to enhance the formability of the primary material in any way.
U.S. Pat. No. 3,753,788 discloses a ferritic stainless steel, such as type 434, containing boron and niobium. This U.S. patent describes that niobium and boron are effective for enhancing freedom of the steel from ribbing. Also, the U.S. patent states that boron is effective for restricting the segregation tendency of the ingot structure of the steel and production of coarse grains.
Generally speaking, the phenomenon represented by the term "ribbing" is close in appearance and feature to that of the term "ridging". However, the phenomenon represented by the term "ribbing" should be distinguished from the phenomenon of the term "ridging". The phenomenon of ribbing is created on the steel due to tension applied to the steel during a cold rolling procedure. The phenomenon of ridging is generated on the steel when the steel is pressed after the steel is finally annealed. That is, the phenomenon of ribbing is produced at a smaller degree of processing than that of the phenomenon of ridging. Ribbing may be easily eliminated by adding boron and niobium to the steel in accordance with the U.S. patent so as to refine the cast structure. However, ridging, which is produced at a large degree of processing, cannot be eliminated by the method of the U.S. patent.
Also, ferritic stainless steel should be provided with excellent resistance to ridging in addition to superior deep drawability. To enhance the deep drawability, it is necessary to control the texture of the steel which has been finally annealed. It is impossible to enhance the deep drawability only by refining the cast structure of the steel.
Japanese Unexamined Patent Publication No. 52-717 discloses a ferritic stainless steel containing aluminum and titanium. In this steel, the amounts of carbon and nitrogen are restricted. The invention of the Japanese unexamined patent publication intends to improve the resistance of the steel to ridging and to decrease the surface unevenness of the processed steel. However, since the amounts of carbon and nitrogen are small, the resistance to intergranular corrosion of the welded portions of the steel is unsatisfactory.